The present invention relates to a method of forming an article via injection of plastics material into a mould.
Various methods of forming thermoplastics materials into articles are known:
1. Injection moulding involves injection of the material into a mould, which is filled. The material solidifies in the mould and takes up its shape. The general rule is that the wall thickness of the article, that is the gap between opposite parts of the mould should be uniform. There is a practical limitation on the wall thickness, in that it is difficult to mould walls thinner than the order of 0.3 mm. With thinner wall thickness, the mould becomes very difficult to fill completely and reliably. This is because a long thin passage for the material exists from the injection port into the mould to the opposite end of the article. The so called flow length: thickness ratio is too high. The result is that injection moulding is impractical and/or uneconomic for certain classes of product, such as vending machine cups.
2. Injection/blow moulding is a process whereby the wall thickness of an article initially formed by injection is reduced by blowing the article to a large diameterxe2x80x94or other cross-sectional dimension. In an injection/blow moulding process, the initially formed articlexe2x80x94the preformxe2x80x94is indexed from the original cavity to another cavity into whose shape it is to be blown. The process of indexing, coupled with adequate cooling of the preform before opening of the initial cavity away from the core results in the preform being likely to be too cool for blowing immediately on introduction into the second cavity. Further, cooling of the preform gives it stability for the indexing. After indexing, a delay is necessary before blowing with gas pressure at the core side, to allow for heat soak from the core to warm the preform again to a plastic state. Consequently, injection/blow moulding is a relatively slow process, but nevertheless widely used for bottle production.
3. An alternative, especially for wide mouth containers is for the material to be thermoformed. In this process, an extruded sheet is peripherally gripped and blown into a cavity. The process stretches the material and a wall thickness less than that practical with injection moulding can be achieved. Usually the result is an uneven wall thickness, with a thicker wall in a base of the article and a thinner wall in its sides. Further there is liable to be wastage at the periphery and in gaps between successive articles formed from the extrusion.
The object of the present invention is to provide an improved method of forming a plastics article.
According to a first aspect of the invention there is provided a method of forming a plastics material article, consisting in the steps of:
forming an injection moulded preform between a pair of complementary mould parts, which define an initial mould cavity for injection of the preform,
separating the one of the complementary mould parts (hereinafter xe2x80x9cthe Substitutable Mould Partxe2x80x9d) from the preform,
replacing the Substitutable Mould Part with one or more replacement mould parts (hereinafter xe2x80x9cthe Replacement Mould Part(s)xe2x80x9d) to enlarge the mould cavity so as to allow stretching of the preform,
stretching at least part of the preform away from the other of the complementary mould parts (hereinafter xe2x80x9cthe Base Mould Partxe2x80x9d) for the forming of it to a finished article shape against the Replacement Mould Part and
removing the Replacement Mould Part(s) for release of the finish formed article.
Normally, the Base Mould Part will be a core part and the Substitutable Mould Part will be a cavity part. However, it can be envisaged that these two parts could be two identical parts to which the core and cavity terminology would not be appropriate.
The method of the invention differs from the conventional injection/blow moulding process in that the mould parts are not indexed as such. In an indexing mould tool, the preform is withdrawn from the initial cavity and moved to the second cavity, whilst simultaneously another core part is introduced into the first cavity part. Injection and blowing occurs simultaneously at the two cavities. Whilst this may seem to have advantage in producing a finished article for each indexing step, the cycle time is determined by the time required for cooling the preform, indexing and reheating the plastics material to plastic state after cooling for indexing. In the method of the invention, the first cavity mould part will normally be separated early, the second cavity assembled without delay and the preform stretched quickly. It is anticipated that the cycle time will be of the order of 2.5 seconds, in comparison to a typical 5.0 seconds cycle for conventional injection blow moulding.
As explained in more detail below, the Replacement Mould Part(s) may be a plurality of individual pans assembled to form the enlarged cavity or a single Replacement Mould Part for the or each enlarged cavity, the part being moved bodily into position. It is anticipated that the former arrangement will be faster in terms of bringing the second cavity around the preform, but the overall cycle time with the latter arrangement is likely to be quicker, especially where final cooling and/or temperature stabilisation for crystallisation occurs in the enlarged cavity after removal of this from the core.
Whilst the degree of stretching may be greater or less, the stretched portion of the preform will normally be stretched by between a factor of 2:1 and 4:1.
A significant advantage of the invention is that it results in controlled bi-axial orientation of the article or at least its blown portion. The stretching involves axial strain and orients molecules of the plastics material in the direction of stretching. The blowing, involving as it does radial expansion hence circumferential strain, strains the material orthogonally to the axial strain. Hence the bi-axial orientation.
In the method of the invention, the preform is unlikely to move transversely of the machine, although due to the construction of the tool, it is likely to move axially, particularly where injection has been via an injection gate in the Substitutable Mould Part and where the mould tool is a multi-impression stack tool. The preform remains in contact with the Base Mould Part at a temperature suitable for plastic extension until after separation. Further, because there is axial movement only of the preform, the Substitutable Mould Part can be removed early, as soon as the plastics material has been cooled, preferably mostly by this part, to a sufficient extent to hold its shape. The material skins against the Substitutable Mould Part at least prior to withdrawal of the latter. However, the material is unlikely to be solid throughout its thickness (which, it should be remembered is about to be reduced) with the central region of the wall remaining hotter and more flexible. Thus heat soak is able to occur into the skin rendering the entire wall sufficiently flexible to stretch into the Replacement Mould Part.
Normally the method will include injection of gas between it and the Base Mould Part to separate at least part of the preform from the Base Mould Part. The stretching of the preform, or at least part of it, can also be effected by injection of gas between the preform and the Base Mould Part.
Preferably, the stretching of the preform is effected by lifting a portion of the preform from at least part of a main piece of the Base Mould Part by a movable piece of the Base Mould Part. Normally the lifted portion of the preform will be moved by the movable piece of the Base Mould Part as far as a corresponding piece of the Replacement Mould Part(s), and the lifted portion of the preform will be captivated between the movable piece of the Base Mould Part and the corresponding piece of the Replacement Mould Part(s) at the end of the stroke of the movable piece. This has the advantage of enabling moulded features to be reproduced in the basexe2x80x94or equivalent partxe2x80x94of the article. For instance, the base may be formed in such a manner as to ensure that it stands stably without the centre being depressed. The captivation of the lifted portion of the preform can temporarily seal apertures in the lifted portion. This can have particular advantage in the case of plant pots, whereby drainage apertures can be moulded in (obviously the preform could not be blown without some means of obturating the drainage apertures).
The injection of gas can be started before the movable piece of the Base Mould Part is moved. Again, the injection of gas can be started before the replacement with the Replacement Mould Part(s) is complete. Whilst the preform can be attached to a main piece of the Base Mould Part by virtue of this piece being polished, at least locally; the preform will normally be is stretched from a portion of it temporarily captivated by the Replacement Mould Part(s), with the stretching being to substantially the final shape of the finish formed article.
In some embodiments, the stretched portion of the preform will be urged into contact with the Replacement Mould Part(s) for its final temperature control, to bring the article to sufficient rigidity for removal, by application of increased gas pressure on the Base Mould Part side of the preform (which may be from gas applied for lifting and/or stretching of the preform). In other embodiments, an alternative of reduced gas pressure on the Replacement Mould Part(s) side of the preform will be used. The final temperature control will normally be cooling, but in the case of CPET material, it will be holding of the article at elevated temperature for crystallisation to occur.
In one alternative of a particular feature of the invention, the enlarged mould cavity is defined by a plurality of Replacement Mould Parts. In this alternative, the enlarged mould cavity can be assembled by radial movement of the Replacement Mould Parts into their replacement position; or it can be assembled by pivotal movement of the Replacement Mould Parts. Alternatively, the enlarged mould cavity can be defined by a single, or one per impression where the tool has multiple impressions, Replacement Mould Part moved bodily into its replacement position.
The Replacement Mould Part(s) will normally be provided with ducts for temperature control fluid and the article is brought to the temperature required for it to be sufficiently rigid for its removal by passage of temperature control fluid through the ducts after stretching and prior to opening of the Replacement Mould Part(s).
The entire sidewall(s) may be stretched. Alternatively part, typically including a stacking feature, may be moulded to final shape in the preform and captivated in the Replacement Mould Part(s) before stretching, so as to avoid this part being stretched. Again, it can be envisaged that the stretching by the movable piece of the core part may be completed before final positioning of the Replacement Mould Part(s), which are shaped to delimit a rim of the sidewall which can be blownxe2x80x94as in the basic inventionxe2x80x94to larger diameter, enabling production of an overhang for instance for stacking.
The plastics material used in the invention may include blowing agent, whereby the sidewall(s) and possibly the base are allowed to expand by foaming. The resultant base and sidewall(s) will be stiffer, in comprising two interconnected skins, than if they were of the same amount of material provided as a thinner solid wall. Use of blowing agent enables a rim of the article and/or a de-nesting feature to be expanded by foaming.
According to another aspect of the invention, there is provided a tool for forming a plastics material article in accordance with the first aspect of the invention, the tool comprising:
a Base Mould Part, preferably a core part,
a Substitutable Mould Part, preferably a first cavity part,
the Base and Substitutable Mould Parts defining an initial mould cavity between them for injection moulding of the preform and being axially movable for separation of the Substitutable Mould Part from the preform,
one or more Replacement Mould Parts movably carried by the Base Mould Part or the Substitutable Mould Part, for movement from a withdrawn position to an advanced position in which it or they provide an enlarged mould cavity defining the outside shape of the finish formed article.
Preferably the tool includes a movable rim forming member, which is slidably mounted on the Base Mould Part for movement, from a rest position in which the Substitutable Mould Part abuts it for moulding of a rim of the article and in which the Replacement Mould Part(s) abut it for captivation of the rim on stretching of the preform, to an advanced position for ejecting the finish formed article.
Further, the Base Mould Part preferably has a movable piece, for lifting a portion of the preform from the Base Mould Part, the movable piece being adapted to seal with a main piece of the Base Mould Part on injection of plastics material. Normally, the movable piece will have a face shaped complementarily with both the opposite face of the Substitutable Mould Part and the Replacement Mould Part(s), whereby the liftable portion of the preform is moulded to final shape in the initial mould cavity. The mould parts can be shaped to form one or more apertures in the liftable portion.
Also, the mould tool preferably includes a pressure gas connection internally of the said main piece and the liftable piece of the Base Mould Part, whereby pressure gas can be introduced on the Base Mould Part side of the preform on lifting of the liftable piece via an aperture in the said main piece normally closed by the liftable piece.
As indicated above a plurality of Replacement Mould Parts can be translationally or pivotally mounted on the Base Mould Part for movement to form the enlarged cavity, with means being provided for moving the Replacement Mould Parts between their withdrawn position and their advanced position.
In the case of pivotal mounting of the Replacement Mould Parts, the means for moving the Replacement Mould Pans can include a slidable member mounted on the Base Mould Part and respective connecting rods connecting the slidable member and the Replacement Mould Parts, the slidable member and the rods being arranged for advance and withdrawal of the Replacement Mould Parts between a withdrawn position in which the Substitutable Mould Part can close on the Base Mould Part and is an advanced position in which the Replacement Mould Parts can close on the Base Mould Part.
In the case wherein a single, or one per impression where the tool has multiple impressions, Replacement Mould Part is translationally mounted on the Substitutable Mould Part or on the Base Mould Part for bodily movement to provide the enlarged cavity, the mould tool includes:
a first slide and actuator carried on the Substitutable Mould Part or on the Base Mould Part, the slide extending at least substantially radially of a centreline of the mould tool,
a Replacement Mould Part carrier slidably mounted on the slide under control of the actuator and
a second slide and actuator carried on the carrier and extending at least substantially parallel to the centreline of the mould tool, the Replacement Mould Part being mounted on the second slide under control of the second actuator.
Further, the mould tool preferably includes a vacuum duct in the Replacement Mould Part extending from an external vacuum connection point to at least one internal vacuum point opening into the enlarged cavity, whereby the finish formed article can be held in the enlarged cavity on withdrawal of the Replacement Mould Part from the Base Mould Part.
This mould tool is preferably provided, in combination with a control system adapted and arranged for:
actuation of the first actuator for inwards radial movement of the carrier and Replacement Mould Part after opening of the Substitutable Mould Part to align the Replacement Mould Part with the Base Mould Part and outwards after stretching of the preform
reciprocating actuation of the second actuator for axial movement of the Replacement Mould Part into abutment with the Base Mould Part for stretching of the preform and out of abutment after stretching of the preform.
According to a third aspect of the invention there is provided a finish formed article made in accordance with the method of the first aspect of the invention.